NXP Semiconductors MC9S12G - Baud Rate Generation

Serial Communication Interface (S12SCIV5)
MC9S12G Family Reference Manual Rev.1.27
NXP Semiconductors 671
20.4.4 Baud Rate Generation
A 13-bit modulus counter in the baud rate generator derives the baud rate for both the receiver and the
transmitter. The value from 0 to 8191 written to the SBR12:SBR0 bits determines the bus clock divisor.
The SBR bits are in the SCI baud rate registers (SCIBDH and SCIBDL). The baud rate clock is
synchronized with the bus clock and drives the receiver. The baud rate clock divided by 16 drives the
transmitter. The receiver has an acquisition rate of 16 samples per bit time.
Baud rate generation is subject to one source of error:
• Integer division of the bus clock may not give the exact target frequency.
Table 20-16 lists some examples of achieving target baud rates with a bus clock frequency of 25 MHz.
When IREN = 0 then,
SCI baud rate = SCI bus clock / (16 * SCIBR[12:0])
Table 20-16. Baud Rates (Example: Bus Clock = 25 MHz)
Bits
SBR[12:0]
Receiver
Clock (Hz)
Transmitter
Clock (Hz)
Target
Baud Rate
Error
(%)
41 609,756.1 38,109.8 38,400 .76
81 308,642.0 19,290.1 19,200 .47
163 153,374.2 9585.9 9,600 .16
326 76,687.1 4792.9 4,800 .15
651 38,402.5 2400.2 2,400 .01
1302 19,201.2 1200.1 1,200 .01
2604 9600.6 600.0 600 .00
5208 4800.0 300.0 300 .00